Challenged by Tom Maschler, of the British publishing firm Jonathan Cape, to "do something no one has ever done before" with a picture book, Williams set out in the 1970s to create a book of paintings that readers would study carefully rather than flip through and discard. The book's objective, the hunt for a valuable treasure, became his means to this end. Masquerade features fifteen detailed paintings illustrating the story of a hare named Jack Hare, who seeks to carry a treasure from the Moon (depicted as a woman) to her love object, the Sun (a man). On reaching the Sun, Jack finds that he has lost the treasure, and the reader is left to discover its location.
Along with creating the book, Williams crafted a hare from 18-carat (75%) gold and jewels, in the form of a large filigree pendant on a segmented chain. He sealed the hare inside a ceramic hare-shaped casket (both to protect the prize from the soil, and to foil attempts to locate the treasure with a metal detector). The casket was inscribed with the legend "I am the Keeper of the Jewel of MASQUERADE, which lies waiting safe inside me for You or Eternity".
Kit Williams later said:
"If I was to spend two years on the 16 paintings for Masquerade I wanted them to mean something. I recalled how, as a child, I had come across 'treasure hunts' in which the puzzles were not exciting nor the treasure worth finding. So I decided to make a real treasure, of gold, bury it in the ground and paint real puzzles to lead people to it. The key was to be Catherine of Aragon's Cross at Ampthill, near Bedford, casting a shadow like the pointer of a sundial."
On 7 August 1979, Williams and celebrity witness Bamber Gascoigne secretly buried the hare's casket at Ampthill Park. Williams announced publicly that his forthcoming book contained all clues necessary to decode the treasure's precise location in Britain to "within a few inches." At the time, the only additional clue he provided was that the hare was buried on public property that could be easily accessed. To ensure that readers from further afield had an equal chance of winning, Williams also announced that he would confirm the first precisely correct answer sent to him by post.
A modified version of the book also appeared in Italian, with a treasure buried in Italy. It was reinvented and translated by Joan Arnold and Lilli Denon with the name Il tesoro di Masquerade (Emme Edizioni).
The book sold hundreds of thousands of copies worldwide, many in the United Kingdom, but also in Australia, South Africa, West Germany, Japan (where the book was called 仮面舞踏会 kamenbutoukai), France and the United States. Searchers often dug up public and private property acting on hunches. One location in England named "Haresfield Beacon" was a popular site for searchers, and Williams paid for the cost of a sign notifying searchers that the hare was not hidden on the premises. Real-life locations reproduced in the paintings were searched by treasure hunters, including Sudbury Hall in Derbyshire and Tewkesbury, Gloucestershire.
In March 1982, Kit Williams received a sketch which he recognized as the first correct solution mailed to him. Williams immediately phoned the sender, "Ken Thomas", a pseudonym of Dugald Thompson, and instructed him to dig for the hare. He realized that Thompson had not solved the puzzle in the intended manner, but appeared at the time to have blundered into a lucky guess. Shortly after Thompson was formally awarded the prize, Williams received a correct solution sent by two physics teachers, Mike Barker of William Hulme's Grammar School and John Rousseau of Rossall School. Barker and Rousseau had seemingly unearthed the prize themselves when digging at Ampthill, but had not noticed it inside its clay box; Thompson discovered it in the dirt piles they left behind.
Bamber Gascoigne, having been asked by Williams to witness the burial of the hare and to document the contest from beginning to end, did so in his book Quest for the Golden Hare. Gascoigne summarized his experiences thus:
"Tens of thousands of letters from Masqueraders have convinced me that the human mind has an equal capacity for pattern-matching and self-deception. While some addicts were busy cooking the riddle, others were more single-mindedly continuing their own pursuit of the hare quite regardless of the news that it had been found. Their own theories had come to seem so convincing that no exterior evidence could refute them. These most determined of Masqueraders may grudgingly have accepted that a hare of some sort was dug up at Ampthill, but they believed there would be another hare, or a better solution, awaiting them at their favourite spot. Kit would expect them to continue undismayed by the much publicised diversion at Ampthill and would be looking forward to the day when he would greet them as the real discoverers of the real puzzle of Masquerade. Optimistic expeditions were still setting out, with shovels and maps, throughout the summer of 1982."
Masquerade's puzzle is elaborate. The answer is hidden in the fifteen painted illustrations. In each painting, a line must be drawn from each depicted creature's left eye through the longest digit on its left hand, and out to one of the letters in the page border. Then from the left eye through the longest digit on the left foot; the right eye through the longest digit on the right hand; and finally the right eye through the longest digit on the right foot. This is only done for eyes and digits that are visible in the painting. The letters indicated by these lines can be made to form words, either by treating them as anagrams or by applying the sequence of animals and digits suggested by the Isaac Newton painting (pictured). Following this method reveals fifteen words or short phrases, which together form a nineteen-word message:
CATHERINE'S LONG FINGER OVER SHADOWS EARTH BURIED YELLOW AMULET MIDDAY POINTS THE HOUR IN LIGHT OF EQUINOX LOOK YOU
The acrostic of these words and phrases reads "CLOSEBYAMPTHILL". Properly interpreted, the message tells the reader that the treasure is buried near the cross-shaped monument to Catherine of Aragon in Ampthill Park, at the precise spot touched by the tip of the monument's shadow at noon on the day of either the vernal or autumnal equinox.
Many additional hints and "confirmers" are scattered throughout the book. For example, in the painting depicting the Sun and the Moon dancing around the Earth, the hands of the two figures are clasped together, pointing at the date of the spring equinox.
On December 11, 1988, The Sunday Times printed a story accusing the winner of the Masquerade contest of being a fraud. "Ken Thomas" was revealed as a pseudonym of Dugald Thompson, and Thompson's business partner, John Guard, was the boyfriend of Veronica Robertson, a former live-in girlfriend of Kit Williams. Guard had allegedly convinced Robertson to help him because both were said to be animal rights activists and Guard promised to donate any profits to the animal rights cause.
The Sunday Times alleged that while living with Williams, Robertson had learned the approximate physical location of the hare, while remaining ignorant of the proper solution to the book's master riddle. After supposedly finding out from Robertson that the hare was in Ampthill, Guard and two assistants were said to have started searching for it using metal detectors. After searching for some time with no success, they drew a crude sketch of the location, which Thompson then submitted to Williams as "Thomas", and it was this that Williams acknowledged as the first correct answer.
Williams was shocked to discover the scandal and is quoted as saying:
"This tarnishes Masquerade and I'm shocked by what has emerged. I feel a deep sense of responsibility to all those many people who were genuinely looking for it. Although I didn't know it, it was a skeleton in my cupboard and I'm relieved it has come out."
Thompson founded a software company called "Haresoft", and offered the jewel as a prize to a new contest which took the form of a computer game, Hareraiser. The company and its game (which many believe to be unsolvable with only meaningless text and graphics), were unsuccessful, yielding no winner.
Robertson has always maintained her innocence in the matter, insisting that she never knew the location of the jewel or even which part of the world it had been buried. Robertson says she had never wanted to know the location of it and was very shocked and upset, not only by the accusations made against her but also by the way she was treated and misquoted by the press at the time.
The hare was auctioned at Sotheby's in December 1988, selling for £31,900 to an anonymous buyer. Williams himself went there to bid, but dropped out at £6,000.
The treasure's whereabouts remained unknown for over 20 years, until it came to light in 2009. The BBC Radio 4 programme, The Grand Masquerade, broadcast 14 July 2009, told the story of the creation and solution of the puzzle. Williams was interviewed and presenter John O'Farrell claimed that this was the first time Williams had talked about the scandal for 20 years. During the interview Williams expressed the desire to see the hare again. Hearing this, the granddaughter of the current owner arranged for Williams to be reunited briefly with his work. This was featured in a television documentary, The Man Behind the Masquerade, which aired on BBC Four on 2 December 2009.
The hare was on display at the V&A Museum, London, as part of its "British Design 1948–2012" retrospective in 2012.
Masquerade became the forerunner of an entire genre of cryptic puzzles known as armchair treasure hunts. It spawned a succession of books and games from other publishers seeking to emulate its success, including The Key To The Kingdom (Pavilion Books, 1982), The Piper Of Dreams (Hodder & Stoughton, 1982), The Secret (Bantam Books, 1982), The Golden Key (William Maclellan, 1982), Treasure: In Search of the Golden Horse (Intravision, 1984) and The Merlin Mystery (Warner Books, 1998). Kit Williams himself also created a second treasure-hunt book, The Bee on the Comb (1984).
Similar hunts have continued being published in various formats. Many later hunts make use of technologies that were unavailable when Masquerade was published, such as Menagerie, an homage to Masquerade, which is web-based, and Text4Treasure, which uses SMS messaging. Others, such as Army Of Zero and West By Sea: A Treasure Hunt that Spans the Globe (Expeditionaire, 2016) follow Masquerade's use of physical media for the main puzzles, but provide additional clues online.
In 2001 the advent of portable and accurate GPS receiving devices started the evolution of a whole new form of treasure hunting called Geocaching, where anyone can hide a physical box or other cunningly disguised container that contains a logbook. They then leave clues as to where they have left the "cache" on the Geocaching.com website for anyone else to find the treasure, sign the logbook and log their finds. This development allows anyone to follow in Kit Williams' footsteps and hide their own treasures for others to find and allows others to search for these treasures. To mark this legacy there is a geocache at the location of the original Masquerade hiding spot.